It is well known in the vehicle art, particularly the automotive art, to provide air cylinders coupled between the frame or body of a vehicle and the axles to adjust the body height above the road in response to varying vehicle loads. In their most common embodiment, such air cylinders are often combined with shock absorbers to provide what are referred in the art as "air shocks". The air shocks combine the damping action of a shock absorber with the load leveling features of an air cylinder. By adjusting the length of the air shock, the body-axle separation and the height of the body above the road may be varied. A typical prior art system is described in U.S. Pat. No. 3,558,155 to Jackson, et al.
The fluid (either liquid or gas) inside the air shock acts as a variable spring. If the vehicle load is increased, the shock is compressed until it balances the added load and the height of the vehicle above the roadway decreases. The vehicle body may be returned to its initial position by forcing more fluid into the shock. Thus, when air shocks are employed, widely different loads may be carried in the vehicle while still maintaining the body at a substantially constant height above the road. This is highly desirable.
There is an increasing desire to provide automatic load leveling equipment on vehicles, particularly automobiles, wherein the height of the vehicle body above the roadway is automatically maintained at a constant distance. In order to do this, it is necessary to have some means for detecting the height of the body above the axles so that the fluid pressure or volume in the air shock may be varied as the vehicle load changes to return the vehicle to its predetermined height.
A number of systems have been used in the prior art to sense vehicle position. For example, a mechanical positioning system is described in U.S. Pat. No. 3,510,143 to Carpenter. Systems using electrical switches are described in U.S. Pat. No. 3,610,611 to Elliot and U.S. Pat. No. 4,293,139 to Brown. A height sensing system utilizing electrical potentiometers is described in U.S. Pat. No. 4,659,104 to Tanaka et al. The use of optical devices for sensing vehicle height is described in U.S. Pat. No. 4,017,099 to Hegel et al., and U.S. Pat. No. 4,391,452 to Ohmori. All of these prior art height sensing arrangements suffer from a number of disadvantages well known in the art. Among these disadvantages are comparatively high cost, difficulty of manufacture, and/or poor reliability. Thus, there is an ongoing need for vehicle height sensors and vehicle height control systems having improved properties.
As used herein the words "air shock" or "shock" are intended to refer to all manner of fluid operable pistons, whether the fluid is liquid or gas, whether the air shock is of the variable length or angular displacement type, and whether or not the air shock incorporates a damping means. As used herein and unless otherwise indicated, the words "light" or "optical" or "optically" are intended to refer in general to the portion of the electromagnetic spectrum having wavelengths in the range of about 0.1 to 100 micrometers.